Hello everyone, sorry to have my first post be under dire circumstances but I have not been able to find an answer to my problem. My Harlequin Rasboras have been dying off recently. The dead fish show no outward symptoms. Gills look normal. All fish act normally until death. Background: A few weeks ago I added 3 young small Harlequin Rasboras and 1 Cobra Guppy as well as another Cobra Guppy about a week later to my already established tank that had 6 Harlequin Rasboras, 3 Guppies and around 30 Red Cherry Shrimp (Totals: 9 Harlequin Rasbora, 5 Guppies, 30 Red Cherry Shrimp). Since then I've lost 6 Rasboras, 2 Guppies(1 Cobra, 1 regular) and still have all of my RCS. My take has been established for over a year now, most of that time with 6 Harleys, 3 Guppies and 25-30 RCS. 1 Guppy had been in there for a year, so may have just been old. Typically, all will look, act and eat perfectly normal and then I will wake up the next morning to find one dead. The only thing I can think of is parasites, because I cannot see any outward symptoms on the deceased fish. Most of the dead fish look like they could be living if they weren't laying dead on the bottom. Poop looks normal. Gills normal. The only clue I have is I found one of my Rasboras acting as if it had swim bladder disease (swimming vertically). It began doing that within 1-3 hours of its death without any prior symptoms, which makes me think it is not swim bladder disease. I have not begun any treatments. Does anyone have any advice on what it could be? Parasites? Does anyone have advice on shrimp safe treatments? Seachem Paraguard? Test results from API Freshwater Master Test Kit: pH: 7.6 Ammonia: 0-0.25ppm (Has been 0 during previous tests, maybe slight elevation due to dead fish?) Nitrite: 0 ppm Nitrate: 15 ppm (Looked between 10-20 ppm) Tank Data: -35 gallon hexagonal -Temperature: 76 degrees Fahrenheit -Filtration: Aquaclear 50 -Time established: Over 1 year -25%-33% Water changes usually every 2-3 weeks. Gravel is always vacuumed. -Live christmas moss for the RCS -Black gravel -Receives indirect sunlight exposure over the course of the day, some hood lighting at night before midnight. -Fed once a day with Omega One fresh water flakes (recently changed to Omega One super color flakes after deaths began).